1. Field of the Invention
This invention relates to the speed control of D.C. motors, and more particularly to such speed control using a feedback signal representative of the D.C. motor armature voltage for generating an error signal indicative of the error in the motor speed wherein the motor armature voltage is accurately measured during a time interval in which the motor excitation is delayed or alternatively sampled at an optimum time.
2. Prior Art
U.S. Pat. No. 3,553,551, Digital Speed Control Apparatus issued to Arnold discloses the application of pulses, the frequency of which are representative of the motor velocity, to both a variable time delay circuit and to a switching circuit. A predetermined time delay is used, the length of which corresponds to the time between the pulses of a pulse train representative of the correct motor speed. The delayed feedback pulse train is compared with that of the measured pulse train. The motor is then caused to either speed up or slow down in dependence upon which of the pulses is first sensed. U.S. Pat. No. 3,950,682, Digital DC Motor Velocity Control System, issued to Dohanich, Jr. utilizes a delay to ensure that a counter has had time to respond to a feedback signal, the period of which represents velocity. The motor pulse drive width is adjusted to maintain a constant motor speed.
U.S. Pat. No. 4,288,729, Control System for DC Electric Motor, issued to Anazi et al, discloses a time delay for opening a switching circuit controlling the energization of the motor to eliminate noise components in the power input to the motor. The time delay is preferably approximately one-fourth of the natural oscillation period of the system.
FIG. 1 shows a known typical feedback control loop for controlling the speed of a DC motor. In such a typical feedback control loop the motor armature voltage is compared to a reference voltage representing the desired speed of motor 10 in summator or comparator 12. The error signal E is amplified by amplifier 14 and power amplifier 16, the latter including an SCR controlled power circuit and the necessary firing circuits therefor. Typical waveforms of the SCR drive circuit representing rotation of the DC motor and a stalled DC motor are respectively illustrated in FIGS. 2a and 2b. The complex waveform shown in FIG. 2a is the armature voltage signal that is fed back to the summator 12 of FIG. 1. The only region in the FIG. 2a that shows the actual motor speed is region f. However, it is readily apparent that the average voltage is somewhat higher than the actual voltage produced by the motor armature. Even in FIG. 2b, where the DC motor is stalled, it is apparent that the armature voltage is not zero due to the presence of the SCR pulse excitation signals. The aforementioned voltage errors cause errors in the speed of the DC motor from the desired speed as represented by the reference voltage at summator 12.
One known method of overcoming the foregoing problem is to use a separate tachometer/generator 18 shown in phantom lines in FIG. 1. However, such a tachometer/generator adds cost along with the benefits it provides in DC motor speed regulation.